Reducing Drag in Towed fishing Gears

Transcription

Reducing Drag in Towed fishing
GearsFishing Trials to Evaluate the
Performance of a Trawl
Constructed from T90 (‘turned
mesh’) Netting
Date:
February 2008
Author:
Ken Arkley
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Working with the seafood industry to satisfy consumers, raise
standards, improve efficiency and secure a sustainable future.
The Sea Fish Industry Authority (Seafish) was established by the Government in
1981 and is a Non Departmental Public Body (NDPB).
Seafish activities are directed at the entire UK seafood industry including the
catching, processing, retailing and catering sectors.
ISBN No. 0 903941 89 9
SEAFISH
Research and Development
Author:
Ken Arkley
Date:
February 2008
Reducing Drag in Towed Fishing GearsFishing Trials to Evaluate the Performance of a Trawl
Constructed fromT90 (‘turned mesh’) Netting
Summary:
This report describes a demonstration trial of a single-rig, demersal whitefish
‘Rockhopper’ trawl constructed entirely of T90 or ‘turned mesh’ netting. This is the
first time that T90 technology has been used in this way in the UK.
The trawl used for this trial was designed and constructed by Icelandic trawl
manufacturer, Fjardanet which has been pioneering this technology for a number
of years. Descriptions of the fishing gear used are included. The report describes
some background to the development work and the concept of T90 technology.
Eight days of commercial fishing trials were carried out in January 2008 using the
Shetland based vessel Mizpah operating on local fishing grounds about 50 miles
NE of Lerwick. Despite being hampered by poor weather a total of 21 hauls were
completed.
The aim of the trials was to evaluate the performance of the T90 trawl with
reference to fuel savings as a result of the reduced netting drag associated with
this technology. This was done by measuring the main gear performance
parameters and comparing them with those of the vessel’s existing gear of the
same general dimensions.
Some catch sampling was undertaken to examine other reported attributes of T90
trawls such improved catch rates, size selection and catch quality.
From a gear performance perspective the T90 trawl compared well with the
vessel’s own trawl. The information gathered on the fuel efficiency aspects of the
gear however did not show any significant benefits from the T90 trawl despite
indications that the netting drag had been considerably reduced, (~20%).
The findings from the catch data were inconclusive. There were some indications
of larger size ranges of some species being caught and retained by the T90 trawl
but the findings did not appear to bear out the findings and experiences of the
Icelandic fishermen to the same extent. There was more loss of marketable size
grades of some species, particularly whiting associated with the T90 trawl. This
was thought to be as a result of the more consistent mesh opening noted
throughout the T90 trawl. The positive side of this was that there were no discards
recorded. There was no noticeable difference in catch quality detected.
The results showed that the combination of the T90 trawl fitted with a conventional
diamond mesh codend of the same mesh size produced the best commercial
results.
The results were insufficient to draw any firm conclusions on the overall
effectiveness of the T90 trawl and a number of proposals for further work have
been highlighted.
Table of Contents:
1.
2.
Introduction ..................................................................................................... 1
Background..................................................................................................... 2
2.1. Mechanical properties.............................................................................. 3
2.2. Selectivity properties................................................................................ 5
2.3. Catch quality aspects............................................................................... 7
3. Approach ........................................................................................................ 9
3.1. Gear details ........................................................................................... 10
3.2. Fishing Trials ......................................................................................... 18
4. Results.......................................................................................................... 21
4.1. Gear measurements .............................................................................. 21
4.2. Catch Results ........................................................................................ 23
4.2.1.
Discards ......................................................................................... 23
4.2.2.
Retained Catch............................................................................... 25
4.2.3.
Catch Quality .................................................................................. 27
5. Discussion .................................................................................................... 28
5.1. Gear performance.................................................................................. 28
5.2. Catch Results ........................................................................................ 29
6. Further Work................................................................................................. 31
7. Acknowledgements....................................................................................... 32
8. References ................................................................................................... 33
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1. Introduction
The catching sector of the UK fishing industry, through the two main national
federations, (National Federation of Fishermen’s Organisations, NFFO and the
Scottish Fishermen’s federation, SFF) have asked Government to collaborate in
developing practical ways of assisting the UK fleet to survive the transition to the
new era of high fuel costs.
On a wider scale, the European Commission has been asked by a wide range of
European fisheries stakeholders to support research and development projects into
increasing fuel efficiency in fishing operations. This includes applying new
technologies with a view to decreasing fuel consumption e.g. reducing the overall
drag of fishing gears.
SEAFISH is collaborating with the catching sector to identify ways to reduce fuel
dependency and to help disseminate the results/information throughout the industry.
The project described in this report is part of a SEAFISH programme of work aimed
at reducing the environmental impact of fishing operations. Areas of investigation
include: improvements to fuel efficiency, (by reducing the overall drag of fishing
gears); improving selectivity; reducing benthic impact; reducing wastage and
improving catch quality.
SEAFISH is continually investigating and developing ideas that contribute to
achieving this aim. Technologies that have the potential for addressing more than
one of these areas at any one time are considered ‘value for money’ and worthy of
attention. The use of T90 or ‘turned mesh’ netting for whole trawl construction was
identified as having the potential to meet a lot of the programme aims.
The project concentrated on improving fuel efficiency by examining a means of
reducing the biggest contributory factor to the overall drag of the fishing gear i.e. the
netting itself. In demersal towed gear the trawl netting can contribute up to 50% of
the overall drag of the system.
Although the primary objective was to investigate potential fuel savings through drag
reduction, other attributes of this technology were also examined. The aim was to
demonstrate benefits of the T90 technology to UK skippers as a viable approach to
improving the efficiency of their gear. This is from the point of view of reducing drag,
whilst at the same time demonstrating the potential of the technology for improving
selectivity and catch quality.
T90 technology takes advantage of increased mesh opening for a given mesh size
when compared with conventional netting configurations. This results in the use of
less netting material for a given size of fishing gear, hence the potential for
reductions in drag. The selectivity benefits of using ‘turned mesh’ or T90 netting
have been acknowledged for some years now. Other benefits such as improved
water flow and catch quality have been investigated more recently.
The T90 technology has been developed for whole trawl application by the Icelandic
trawl manufacturer Fjardanet. Fjardanet is based in Akureyri in Northern Iceland
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and is part of the Hampidjan group. To date (February 2008), Fjardanet has
produced 21 full trawls and supplied numerous trawl sections constructed from T90
netting to Icelandic skippers. Icelandic experiences have demonstrated improved
catches, improved catch quality and fuel savings as a result of the reduced gear
drag. The commercial success of the Fjardanet trawl designs and the experiences of
Icelandic fishermen using them, prompted SEAFISH to investigate this technology
for UK application.
2. Background
The concept of ‘turned mesh’ or T90 technology is not new. The selectivity of T90
netting has been investigated since the late 1980s. Previous investigations have
concentrated on the application of this netting configuration for use in codends and
extension sections as a means of improving size selectivity. More recently, the
emphasis has changed to investigating the drag characteristics of this material and
the potential fuel saving as a result of incorporating the netting into other sections of
a trawl.
The Icelandic net manufacturer Fjardanet based in Akureyri, supported by its parent
company Hampidjan took the development a number of stages further by producing
a full commercial trawl design in T90 netting.
Fjardanet has been working with ‘turned mesh’ netting for the construction of both
demersal and pelagic trawl codends, extensions and belly sections for a number of
years. It was their net designer and marketing manager, Hermann Gudmundsson
that came up with the idea and the design for a full trawl constructed in T90 mesh.
This was following their experiences and positive feedback from skippers using T90
netting in the bellies and codends of groundfish trawls targeting cod. They found
that the new designs filtered out large proportions of undersize cod and the nets had
the added benefit of being easier to tow.
Following encouraging results from scale model tests in the SINTEF Flume Tank in
Hirtshals, Denmark, Fjardanet was confident enough to go into full scale trials which
were conducted on board the Icelandic Marine Institute’s research vessel (FRV Arni
Fridriksson).
Supported by underwater observation and filming of the gear in operation, the trials
enabled minor adjustments and modifications to be made to the prototype trawl prior
to full commercial evaluation. This was carried out by putting the trawl on board a
trawler belonging to the Icelandic Fish capture/processing company Samherji HF on
a ‘sale or return’ basis.
The skipper was completely satisfied with the performance of the trawl and
compared to the vessels standard gear caught more and better quality fish.
The skipper commented on the ease at which the net was towed compared to a
standard trawl of the same headline/groundgear length. This is not surprising
considering that the amount of netting required for the construction of a T90 trawl is
significantly less than with standard diamond meshes. Approximately one-third less
netting is required, thus reducing the twine surface area and drag of the netting.
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Fjardanet continues to produce full T90 groundfish trawls for Icelandic skippers with
more and more showing considerable interest.
2.1. Mechanical properties
The introduction of new netting configurations for the construction of fishing gear, for
whatever reason; (improving selectivity, reducing netting drag etc.) invariably raises
questions from gear technologists and fishermen alike about the practicalities and
usability of the new materials. The main concern is usually that of strength.
With T90 netting, which results in changing the direction of stretching forces acting
on the turned meshes, the stability of the meshes must be established as being at
least comparable with that of conventional netting if it is to be acceptable as a
practical alternative. Knot slippage resulting in changes in mesh size and mesh
deformation is unacceptable from the point of view of both selectivity and gear
performance.
Standard
diamond
mesh
netting
T90 mesh
netting
Photograph courtesy SINTEF
Turning meshes through 90º does not result in the decrease of their resistance to breaking
force. An increase of tensile force has been registered of about 10% to 25% for meshes of
1.1mm to 1.8mm bar thickness, respectively.
Trials (1 Moderhak 2000) have demonstrated that the common perception that turned
mesh netting is weaker, (lower resistance to tensile force) than conventional netting
is not the case. As expected, the resistance to tensile force varies with twine
diameter but is not affected by mesh bar length. However, factors such as material
type, quality and treatment do influence tensile strength. The knots used in the
construction of the netting play an important role in the process of stretching as they
act as a sort of ‘shock absorber’ delaying the point at which the mesh twine breaks.
It appears that the knot in the ‘turned mesh’ configuration provides better resistance
to the shearing forces involved in mesh breakage.
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More recent testing (2 Moderhak and Niemiro 2005) confirmed earlier findings (1
Moderhak 2000) indicating increased breaking strength of T90 meshes as compared
with standard meshes in the range of 10% - 25% depending on netting material and
twine diameter. Turning conventional netting through 90º increases the elasticity of
the mesh, in particular netting constructed of PE twine.
The wider opening of the meshes achieved by the T90 configuration, (due in part to
the construction of the knot and partly to the orientation of the mesh bars as they
come away from the knot), results in a codend with a relatively large circumference
and cross-sectional area. This produces a much more stable codend in a water flow
when compared to narrow, standard diamond mesh constructions.
(1) Moderhak, W. 2000. Preliminary investigations of the mechanical properties of meshes turned
through 90º. Bull. Sea Fish. Inst., Gdynia, 1 (149): 11-15
(2) Moderhak, W. and T. Niemiro 2005. Mechanical properties of twine and netting used in Baltic
fisheries. Bull. Sea Fish. Inst., Gdynia, 3 (166): 27-37
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2.2. Selectivity properties
Research vessel and commercial fishing vessel trials to examine the selectivity of
T90 codends have demonstrated improved selectivity is possible for certain species
such as cod (3 Moderhak 2000). There are also indications of other benefits such as
the ability to capture greater numbers of marketable fish (cod). Reports from
commercial experiences (Icelandic fishermen) support these observations.
Improved water flow through netting meshes, particularly in the region in and around
the codends may significantly improve the selectivity of the gear by stimulating fish
inside the codend to follow the increased water flow out through the meshes.
Underwater observations of fish behaviour seem to support the idea of changes to
water flow (increased speed) influencing fish reactions to the gear. The two main
responses observed are: 1) the fish will tend to turn to face the increased water flow
and try to maintain station until exhausted, at which point they fall back towards the
end of the net, or : 2) turn with the flow towards the increased outflow through the
meshes. At this point, fish size relative to mesh opening will determine whether the
fish escapes or is retained.
From a selectivity perspective it is therefore advantageous to be able to manipulate
or alter this flow pattern. In simple terms the aim is to allow a greater amount of
water to enter the codend and/or to make the passage of water out through the
netting easier, the simplest way being to increase and maintain mesh opening in the
netting. Increased flow and reduced pressure will help indicate potential escape
routes for fish. Investigations on fish (cod) behaviour have shown that increased
radial flow rates in certain areas of the trawl (codends in particular) result in greater
numbers of fish escaping.
Fish escaping through smaller meshes of a greater opening would sustain less
skin/mucous/body damage than those passing through larger, closed meshes.
(3) Moderhak, W. 2000. Selectivity tests of polyamide and polyethylene codends made of netting with
meshes turned through 90º.
Bull. Sea Fish. Inst., Gdynia, 1 (149): 17-25
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It is possible for a fish of a given size to
pass through T90 mesh of a mesh size
compatible with its body shape and size
without incurring injury/damage caused by
forces exerted on the fish by the mesh bars
(1)
1
Half size
T90
mesh
2
3
Even by doubling the standard diamond
mesh size, the same size fish would still
encounter similar problems. However,
assuming the same twine constructions
throughout the force required to open the
mesh would be reduced (3)
Half size
Standard
diamond
mesh
Large
standard
diamond
mesh
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The same size fish attempting to pass
through the same mesh size in standard
diamond mesh configuration would have to
work against the forces exerted by the
mesh bars resulting in removal of
protective body slime and/or injury and
without guarantee of release (2)
Large
standard
diamond
mesh
indicating
smaller fish
size to
enable
unhindered
passage
LargeT90
mesh
indicating
larger fish
size
capable of
unhindered
passage
6
On this basis the size of fish which would
be able to pass through the larger standard
diamond mesh with the same level of ease
would be considerably smaller (4)
The equivalent larger mesh size in T90
configuration would allow passage of a
much larger fish without incurring damage
and/or injury (5)
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2.3. Catch quality aspects
Deterioration of fish quality starts from the point of capture, significantly in the
area of containment namely the codend. There are often obvious visual
indicators of this in the form of scale removal, skin damage and crush injuries
etc. However, other damage such as sub-cutaneous bruising, pressure
damage to flesh, internal bleeding etc. may not become apparent until the
later stages of processing, and by which stage the shelf-life has already been
significantly reduced.
The factors that influence this process are, amongst others; the quantity and
type of catch retained in the codend; the turbulence from water flow within the
codend; the movement of the codend as a result of water flow in and around
the codend; the size, rigging and construction material of the codend itself;
and the length of time that the catch is exposed to these conditions, i.e. tow
duration.
Clearly it is advantageous from a quality perspective to try and keep captured
fish alive throughout the whole fishing process, from net to vessel. From a
gear technology point of view it is therefore desirable to try and modify gear
designs to reduce the effects described above which can have a negative
affect on the condition of the fish.
Underwater observations of fishing gears have revealed that codends
undergo a lot of vertical and lateral motion, undulating up and down and
swaying from side-to-side during the duration of a tow. This creates a lot of
the unfavourable conditions for fish within the codends that result in the
problems previously identified.
Work has been done (5Hansen et al. 1996, 6Hansen 2004) to develop codend
designs to reduce these adverse effects. Some of the most effective
developments have involved the use of T90 netting.
When compared to standard diamond mesh codends, the swinging and
oscillation movements of T90 codends during towing are much less apparent.
The subsequent motion of the catch within the codend is also significantly
reduced (6Hansen 2004). Changes in towing speed and catch size affect the
codend motion and turbulence within the net to a lesser extent than for
standard codend designs. With standard netting, as the catch increases there
is an increase in drag on the codend. Model experiments indicate that T90
constructions show only marginal or no increase in drag with increasing catch
load (6Hansen 2004).
Improving the conditions within the codend to reduce damage to the catch can
clearly have benefits for retained catch quality. These benefits should also
apply to escapees. It is reasonable to assume that under improved conditions
those fish able to escape should have a better chance of survival thus
reducing incidental mortality.
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Both commercial experiences (Icelandic fishermen) and scientific research
(1 Moderhak 2000) have indicated benefits for catch handling and quality. The
condition of the fish in catches from T90 trawls and codends, even from
relatively long tow times, has been observed as being significantly and
consistently better than that from standard trawls. The catch remains alive for
longer when compared to standard trawls. This is related to better water flow
through T90 trawls and codends due to the more open nature of the T90
netting. This affects the flow parameters such as water velocity and flow
intensity through the netting in general and in particular in the codend
(4Moderhak 1993). This improved water flow is thought to influence the size
composition of the fish retained. The larger, generally stronger swimming fish
are encouraged into the codend region of the trawl more quickly in this
improved flow environment. There appears to be less tendency for the fish to
swim forward in the trawl and therefore reduces chance of escape.
Section of a T90 extension showing consistent mesh opening
Another practical benefit of this improved flow is that catches have fewer
tendencies to be spread through the main body or the extension of the trawl.
This reduces catch handling time and risk of damage by removing the need to
shake fish down to the codend prior to emptying.
(4) Moderhak, W. 1993. Some problems of water flow through trawl codends.
ICES C.M. 1993/B: 11
(5) Hansen, U.J., L.H. Knudsen, P. Nielsen & E.M. Andersen, 1996: Development of fishing
gear for Fishing Vessel 2000
Danish Institute for Fisheries Technology and Aquaculture (confidential)
(6) Hansen, U.J. 2004. Performance of a trawl codend made from 90º turned netting (T90)
compared with that of traditional codends.
ICES FTFB WG meeting, Gydynia, April 20 – 23, 2004
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3. Approach
In 2007 SEAFISH Gear Technologists approached the Icelandic net
manufacturer Fjardanet to obtain information on their development work with
T90 trawls and to gather evidence of the commercial performance of their new
gear.
During a visit to Fjardanet in Akureyri, Iceland, SEAFISH Gear Technologists
set up a collaboration arrangement with Fjardanet’s net designer currently
pioneering this technology, to design and construct a whitefish T90 trawl
based on a UK trawl design, to be trialled by a UK vessel.
The objective was to demonstrate the T90 concept to UK skippers by
transferring the Icelandic technology and evaluating it as a viable means of
reducing towed gear drag and assessing its potential for improved selectivity,
catch quality and for application to other gear designs and fisheries. Some of
these elements were also examined as having potential to improve gear
designs for the targeting of squid.
Working with Fjardanet’s gear designer and Marketing Manager Hermann
Gudmundsson, SEAFISH gear technologists designed a demonstration trial of
the T90 technology.
The first stage was to identify a suitable vessel and a UK gear design for
conversion to T90. Since Fjardanet’s experience has been almost exclusively
with relatively large and powerful Icelandic trawlers, it was decided that the
SEAFISH trial should focus on the larger horsepower sector of the UK fleet
currently predominantly based in Scotland. SEAFISH put out a request for
interested skippers operating single-rig demersal whitefish trawlers in the
750hp to 1000hp category, that were operating 2-panel groundfish
‘rockhopper’ trawls targeting mixed, demersal finfish species. Following a
tender process in July 2007, the Shetland based vessel MIZPAH (LK173)
skippered by David Robertson was selected to carry out the demonstration
trials.
MIZPAH (LK173):
Built:
Cambletown 1988
Steel, transom stern, full
shelterdeck
Length: 26.88m
Breadth:7.5m
Draft:
3.66m
GRT:
254
Engine: Caterpillar
Power: 578kw/775hp
o
MIZPAH (LK173)
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At the time of the trials the vessel was using trawls produced by Jackson
Trawls of Peterhead, Scotland. Following discussions with Jackson Trawls
outlining the objectives of the project, permission was granted for the
conversion of a Jackson ‘Hopper’ trawl design to T90 configuration.
The brief given to the Fjardanet net designer was to produce a T90 version of
the vessel’s own net allowing a certain amount of design freedom as required
by the T90 concept but whilst remaining within the constraint of maintaining
the overall frame size, (fishingline lengths) of the original design.
As the project was designed to demonstrate possible reductions in the overall
drag of the gear, the net manufacturer was also given some freedom to
modify twine diameters (and mesh sizes to some extent), but without
compromising the integrity of the gear. Due to differences in trawl design and
construction between Icelandic and UK net makers some problems were
encountered with material availability when it came to matching netting
materials, this resulted in some compromises with regard to mesh size and
twine diameters.
Manufacture of the T90 trawl by Fjardanet was completed in December 2006
To try and ensure that the Icelandic gear design would be compatible with UK
fishing conditions, the net was designed to fit on to a groundgear arrangement
typical in size and construction of that being used by the Mizpah.
3.1. Gear details
The general rigging arrangement for the trawls used during these trials is
shown in figure (1).
The trawl doors used were manufactured by the US firm NETS Systems and
were 5m2 in area. The trawl doors were rigged with 5 fathom (9.14m) double
chain backstraps and connected to the trawl with 30 fathoms (54.86m) of
26mm wire single sweep and 30 fathoms (54.86m) of split bridles. The upper
bridle was made up of 18mm wire and the lower bridle of 16mm mid-link
chain.
Identical groundgears were used on both nets, (both manufactured by local
riggers, LHD Ltd.). The groundgear was a typical ‘Rockhopper’ arrangement
as used by most vessels operating in Shetland waters (see figure (2)). The rig
was made up of 3 main sections; mid; shoulder; and wing or toe sections
rigged on 24mm wire.
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The mid-section consisted of 1 x 19’10’’ (~6.0m) length of 24mm wire with 24
x 18’’ (457mm) rubber ‘Rockhopper’ discs. The discs were separated by 1 x
7’’ x 7’’ (178mm x 178mm) rubber spacers. Extra weight in the form of 6 x 2kg
lead washers was evenly distributed over the length of the centre section.
‘Rockhopper’ groundgear – mid-section
The shoulder sections (x 2) were rigged on the same length of wire, half
rigged with 18’’ (457mm) Rockhopper’ discs (x 12) and half rigged with 16’’
(406mm) discs (x 13). The same additional weights were distributed between
the same size spacers.
‘Rockhopper’ groundgear - Shoulder-section
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The wing sections (x 2) consisted of 16 x 16’’ (406mm) discs with 2 x 7’’x 5’’
(178mm x 127mm) spacers between each disc except for the last two discs in
each section which were separated by one spacer. Additional weight was
added as for the other two sections.
‘Rockhopper’ groundgear – Toe or Wing-section
The toe-end of the groundgear was finished off with a ‘bumper-bobbin’
arrangement forming the connection to the groundgear extension chains (2 x
~10.5m x 16mm mid-link) via a 12’’ ‘bouncer pin’. (See photos for details).
‘Bumper’
bobbin
arrangement
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‘Bouncer pin’ detail
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The groundgear was attached to the fishingline of the net using 8mm, 3-strand
PP/PE rope fastened through the top hole of the ‘Rockhopper’ discs.
‘Rockhopper’ attachment detail
Both trawls were rigged with the same floatation, namely 58 x 11’’ (280mm)
deepwater trawl floats evenly spaced over the full length of the headline.
These were attached using the same rope used for the groundgear
attachment.
Headline float attachment detail
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Outline schematic diagrams of the two trawls tested during these trials are
shown in figure (3).
The mesh sizes for the vessel’s own net are given as inside mesh
measurements whereas for the Fjardanet design they are given as full mesh
(knot centre to knot centre).
More detailed information regarding the net designs supplied by the
respective net manufacturers has not been included in the report at this stage
as a matter of confidentiality. More detailed information on the T90 design will
be made available at a later stage after further work has been carried out.
From the Figure (3) it can be seen that both nets were based on the same
frame size, i.e. the same headrope and footrope lengths. The T90 conversion
involves a reduction in the amount of material used for the construction of the
trawl and results in changes to the panel taper rates in some of the netting
sections. There were some differences in mesh sizes and twine diameters
between the two designs. Problems with material availability as a result of
differences in commercial mesh sizes between Icelandic and UK
manufacturers resulted in some compromises having to be made. This
resulted in the mesh sizes in some panels not being as closely matched as
had originally been expected.
As compromises with mesh sizes had to be accepted, it was felt acceptable
that the Fjardanet net designer was given the freedom to introduce other
measures that would contribute to the reduction in netting drag. The use of
lighter twine construction was advocated where necessary and/or appropriate.
To this end high performance polyethylene (HPPE) twines were used for the
construction of the T90 netting. This allowed smaller diameter twines to be
used in some areas without compromising the strength of the gear.
Nominal twine surface areas (excluding codends and extensions) for each of
the two nets under test were calculated for comparison. The T90 trawl had
almost 40% less than that of the vessel’s own Jackson net, (104.5m2 and
173.7m2 respectively).
Some of the trawl construction methods used by the Icelandic net maker
differed from those used by the UK makers. Additionally, the T90
configuration required different approaches and methods of construction
compared to standard diamond mesh netting.
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General images of T90 trawl
T90 trawl being prepared for sea trials
Panel join in T90 netting
Centre (crown) section of square
showing panel join
Lower wing netting
Crown meshes in belly panel showing
bolsh attachment to fishingline
Join between extension and codend
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The general codend arrangements used on both trawls are shown in figure
(4). The T90 codend was narrower (~33%) in mesh count as a result of the
T90 construction.
The rigging of the T90 codend was substantially different using a completely
different approach to codend closure. The last section of the codend (four
meshes) was constructed in conventional diamond mesh netting for ease of
closure. The T90 codend was closed/secured using what is termed an
‘Alaska’ knot. This is effectively a ‘chain’ knot or lacing which results in the
codend maintaining a rectangular shape when closed. This is in contrast to
the conventional method which has the meshes tightly gathered together
around the codline.
T90 codends showing conventional diamond meshes
and ‘Alaska’ knot closure
T90 codends with catch onboard Icelandic vessel
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The T90 codend was also rigged with selvedge ropes as per Icelandic
practice.
T90 codends and extension showing selvedge ropes
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3.2. Fishing Trials
The MFV Mizpah was chartered for a period of up to 10 days in January 2008
to carry out sea trials to evaluate the performance and handling of the T90
trawl construction. The trials also incorporated time to carryout observations
on the vessel’s existing conventional gear to establish some baseline data for
comparison with the T90 trawl. The original plan was to collect data on the
vessel’s own net on a separate trip prior to the T90 trials. However, due to
delays with the construction of the T90 trawl, a shortage of time prevented a
separate trial taking place and so both trawls were evaluated on the same trip.
The exercise combined engineering performance trials to measure relevant
gear parameters with some commercial fishing trials to evaluate other aspects
of the gears performance such as selectivity and influence on catch quality.
The T90 trawl was compared to the vessels existing gear of the same/similar
overall dimensions.
The engineering trials entailed the fitting of gear performance monitoring
equipment (SCANMAR) to the gear and underwater cameras as required
and/or conditions allowed. SCANMAR distance sensors were attached to the
wingends of the trawls to measure horizontal mouth opening and on the
centre of the headline to record vertical opening. A speed sensor was also
attached to the headline to record the speed of the trawl through the water.
Distance sensors were attached to the trawl door backstraps to measure the
horizontal distance between the doors. Underwater loads cells were used on
some tows to measure the load exerted by the trawls plus sweep/bridle
arrangements. These were connected at the junction between the trawl door
backstraps and the sweeps.
The commercial fishing trials followed the vessels normal operating practices,
targeting the normal range of groundfish species for which the vessel has
quota. Operations took place on the fishing grounds that the vessel would
normally be fishing on at the time of the trials. The vessels normal gear rigging
arrangement was used (see Figure (1).
The main objective was to demonstrate the fishing performance by measuring
the main gear parameters such as door spread, headline height, wingend
spread, net speed, towing loads and fuel consumption. The other attributes of
the gear were evaluated by monitoring catch rates, catch quality and catch
profiles with particular interest in selectivity and discards. The aim was to see
if the gear could achieve results similar to those observed by Icelandic
skippers who have consistently reported improved catches, improved catch
quality and fuel savings.
Over the period between 18th and 30th January two SEAFISH Gear
Technologists were based in Lerwick in the Shetland Islands to prepare the
trawl and groundgear and carry out the sea trials. During eight days of fishing
a total of 21 tows were carried out, often in very poor weather conditions. At
one stage the trials were halted for a period when the vessel returned to
Lerwick for shelter as a result of severe weather.
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The vessel’s own trawl with standard codend was used on 7 occasions and
compared with 6 hauls with the full T90 trawl. A further 4 tows were
conducted using the T90 trawl fitted with the vessel’s standard diamond mesh
codend/extension and using the vessel’s own trawl fitted with the T90 codend
and extension.
The intention was to monitor all tows for gear performance using a
combination of vessel’s own instrumentation and SCANMAR acoustic net
monitoring equipment. Down time due to equipment maintenance/failure
meant that on some tows some gear parameters were not recorded.
However, sufficient data were collected to enable gear parameters to be
established for most net configurations under examination.
SCANMAR distance sensor on trawl door
SCANMAR load cell measuring
tension behind the trawl doors
A series of tows was set aside for camera work and more specialist
instrumentation (load measurements).
SEAFISH underwater camera
The underwater camera was used on a limited number of tows when and
where conditions were suitable. Due to poor water clarity it was difficult to
obtain much useful film of the gear. Some images of the mouth of the trawl
were obtained giving an indication of the net rigging on the fishingline and
groundgear and showing consistent mesh opening achieved by the T90
netting.
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© SEAFISH
The underwater film confirmed the gears ability to traverse rough ground
without sustaining damage to the net.
Fishing operations took place in an area approximately 50 miles north east of
Lerwick on traditional grounds used at that time of year. The vessel was
targeting mixed groundfish species including cod, haddock, whiting, saithe
and mixed flatfish species
Catch monitoring was carried out on 16 tows comparing the standard Jackson
net (vessel’s trawl) rigged with conventional diamond mesh codend/extension
with the full T90 net, (8 hauls with each configuration). The retained catch
was recorded using the vessel’s existing catch handling practices. All
retained catch was sorted by species and size grade, weighed, boxed and
iced in ~48 kg boxes. All catch weights were recorded by species for each
haul on the wheelhouse computer and logged on a daily basis.
Samples from each size grade were taken to produce a range of fish lengths
per size grade for each 24 hour sampling period, (4 tows). A minimum of 3
samples per grade were taken at each sampling period, (~100kg) for each
species. These data were used for the retained catch information in the
results section.
The discarded element from each monitored haul was sampled and measured
to provide numbers at each length category (1cm) for the main species
discarded.
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© SEAFISH
4. Results
4.1. Gear measurements
The vessel towed at speeds between 2.5 and 3 knots in depths of 70 – 80
fathoms. Warp: depth ratios of between 2.5:1 and 3:1 were used to achieve
the gear measurements recorded.
Table (1) shows the average measurements for the main gear parameters
monitored during the trials.
Table (1): Gear parameters for the trawls under test
Parameters Measured
Jackson
Trawl
Conventional
ext/codend
T90 Trawl
T90 ext/codend
Jackson Trawl
T90 ext/codend
T90 Trawl
Conventional
ext/codend
2.4:1
2.5:1
2.8:1
2.9:1
2.6
2.8
3.0
3.0
79.3
6.2
24.0
92.0
69
6.6
21.3
91.6
77.6
5.5
19.35
/
71.3
6.05
21.95
/
Port
4.0
4.0
4.0
4.1
Stbd
3.95
3.9
4.0
4.0
121.3
122.3
131.7
124.2
1613
*2.22
*2.32
1610
*1.85
*1.8
1621
/
/
1601
/
/
Warp: depth
Speed over ground
(knots)
Depth (Fathoms)
Headline height (m)
Wingend spread (m)
Door spread (m)
Towing load (t)
(SCANTROL)
Fuel consumption
(litres/hour)
Engine revs.
Port
SCANMAR
loads (t)
Stbd
Measurements are averages from 16 representative tows, 4 for each configuration
*Readings taken from two tows only
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© SEAFISH
Door spreads for the T90 and the vessel’s own net were very similar and
indicated bridle angles of approximately 15º. There were slight differences in
the wingend spreads and headline heights between the two nets. The T90
trawl achieved on average 0.4m extra height but showed on average 2.7m
less wingend spread. These figures were consistent with what was expected
for these gears to operate effectively.
Indications of the relative drags of the two gears were obtained from
acombination of measurements. Warp tension readings were obtained from
the vessel’s SCANTROL autotrawl system which gave an indication of the
total loads in the trawling system. SCANMAR underwater load cells were
fitted behind the trawl doors to measure the drag of the trawl net itself. Engine
speed (revolutions per minute) and fuel consumption (litres per hour) were
recorded from wheelhouse instrumentation.
The total loads recorded from the SCANTROL system were consistent and
indicated no significant difference between gears. Similarly, the averaged fuel
consumptions and engine speeds did not indicate any noteworthy changes.
However, the towing loads measured by SCANMAR load cell behind the trawl
doors did show a more marked difference between the T90 and the vessel’s
own trawl. This averaged out at approximately 20% reduction in load in favour
of the T90 trawl.
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© SEAFISH
4.2.
Catch Results
4.2.1. Discards
Discards from both trawls were relatively low from all tows sampled.
Cod:
No cod discards were recorded for either net.
Haddock:
No haddock discards were recorded from the hauls with the T90 trawl and
very few were measured for the standard (control) trawl. All fish below the
minimum landing size (MLS) of 30cm were obviously discarded but the vessel
also discarded some haddock just above MLS which would have been at the
low end of the size range of grade 4.
Haddock Discards from T90 and standard trawls
60
Control
50
Numbers
40
30
20
10
0
10
15
20
25
30
35
40
45
50
Length Class (cm )
Whiting:
Whiting was the only species with any notable numbers of discards but as
with haddock it was only the standard trawl that retained them.
Whiting Discards from T90 and standard trawls
Control
1200
T90
1000
Numbers
800
600
400
200
0
10
15
20
25
30
35
40
45
50
Length Class (cm )
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© SEAFISH
Saithe:
Although some saithe were retained it was generally only the higher grades.
There was discarding of saithe from both trawls but these fish were all above
MLS (30cm). This practice was influenced by market and quota conditions.
Saithe Discards from T90 and standard trawls
Standard
25
T90
Numbers
20
15
10
5
0
10
15
20
25
30
35
40
45
50
Length Class (cm)
Mixed flatfish species:
The numbers of flatfish caught with both trawls were relatively small. For the
purposes of recording catch data the numbers of plaice, lemon sole, megrim
and witch have been grouped together. From the discarded mixed flatfish
species figures it can be seen that the standard trawl was retaining more
small flatfish than the T90 trawl.
Mixed Flatfish Discards from T90 and standard trawls
Control
80
T90
70
60
Numbers
50
40
30
20
10
0
10
15
20
25
30
35
40
45
50
Length Class (cm )
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© SEAFISH
4.2.2. Retained Catch
Cod:
Cod catches
14
Standard Trawl
12
T90 trawl
T90 trawl with diamond codend
10
Standard trawl with T90 codend
Numbers
8
6
4
2
0
40
45
50
55
60
65
70
75
80
85
90
95
Length (cm)
Haddock:
Haddock catches
100
Standard trawl
T90 Trawl
T90 Trawl with 120 Diamond
codend
Standard Trawl with T90 codend
Number of fish
75
50
25
0
31
36
41
46
51
56
61
66
Length (cm)
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© SEAFISH
Whiting:
Whiting catches
180
160
Standard trawl
140
T90 trawl
Number of fish
120
T90 with diamond
codend
Standard trawl with
T90 codend
100
80
60
40
20
0
33
38
43
48
Length (cm)
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© SEAFISH
Table (2): Summary of catch data for trawl configurations examined
Catch by EC
grade
Haddock 1
Haddock 2
Haddock 3
Haddock 4
Total (kg)
Cod 1
Cod 2
Cod 3
Cod 4
Cod 5
Total (kg)
Saithe 1
Saithe 2
Saithe 3
Lythe
Total (kg)
Whiting 1
Whiting 2
Whiting 3
Whiting 4
Total (kg)
Mixed flatfish
inc. Monkfish
Total (kg)
Squid
Total (kg)
Total catch
value (£)
Catch weight (kg) for each trawl configuration examined
Standard
T90 trawl
T90 trawl
Standard
trawl with
with
with T90
trawl with
standard
standard
codend
T90 codend
codend
codend
52.6
91.4
113
87.9
96
83
127.7
78.9
182.2
51.5
143.3
66.6
211.2
28.8
230
22
542
254.7
501
255.4
/
/
/
/
157
43.5
49.5
38
59
10.5
31.5
43
104
78
115
106
48
43
32
25.5
368
175
228
212.5
30
66
157
11.6
42
18
177
105.4
96
/
48
131
/
59.5
112.2/
42.7
168
143.5
494.2
290.7
/
/
/
/
196
39.7
313.3
118.5
156
34.2
141.3
146.5
48
/
43
0.2
400
73.9
497.6
264.8
1
213.1
104.9
127.5
1
55.3
55.3
213.1
7.5
7.5
104.9
/
/
127.5
49.8
49.8
2 384.24
1 338.43
2 391.07
1 899.42
Note: The catch value is based on the same price per kg for each grade of
each species for each trawl configuration examined.
4.2.3. Catch Quality
The quality of the catches from each net was monitored visually by SEAFISH
staff onboard that made some subjective observations primarily relating to
how lively the fish were as they came onboard, appearance with respect to
external damage as a result of interaction with the gear and/or other elements
of the catch. From these observations there was no noticeable difference
detected between the catches from the standard and the T90 trawls.
SR595
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© SEAFISH
5. Discussion
5.1. Gear performance
It must be emphasised that the results obtained from this trial should only be
considered as indicative of the performance of the gears under examination.
The limited amount of data collected and the constraints imposed by the
weather conditions and limitations of the equipment mean that definitive
conclusions can not be drawn from this exercise.
The trials were designed as a demonstration exercise to provide an insight
into the T90 full trawl concept and to gain an indication of the potential of this
technology for further application in other gear designs and fisheries as a
means of primarily improving fuel efficiency and secondly improving selectivity
and catch quality.
The general geometry and performance of the two trawls under test were
considered satisfactory from the point of view of being matched for the
comparative purposes of the trial.
Unfortunately, the information gathered on the fuel efficiency aspects
examined did not appear to support the observations and experiences of the
Icelandic fishermen who have been the forerunners in the development of this
technology. The reduced twine surface area associated with the T90 design
(~40%), when compared to the vessels own standard trawl of the same
general size was expected to produce a noticeable reduction in the overall
drag of the trawl system. The only indication of drag reduction was given by
the SCANMAR load cells reading the tensions behind the trawl doors.
Although these reductions were quite large (~20%), this result came from a
small number of tows where this parameter was recorded and therefore may
not be reliable. This drag reduction was not supported by the total load
readings obtained from the SCANTROL system or reflected in the relative fuel
consumptions associated with each gear type.
Considering the variations in conditions experienced during the trials
(weather, towing speeds etc.) it may be reasonable to suggest that the
indications from the fuel consumption measurements are not representative or
reliable over such a short period of time (insufficient data/samples).
General observations of SEAFISH staff and crew indicated that from a gear
handling perspective, the T90 trawl posed no additional problems or concerns
compared to standard gears.
Some of the crew commented on the lighter twine constructions used in the
T90 net and questioned whether it would be as strong as there normal
construction. Over the duration of the trial there were no problems associated
with these differences.
SR595
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© SEAFISH
5.2. Catch Results
Every effort was made to maintain similar conditions and fishing grounds,
species mix etc. for the duration of the catch sampling exercises. It must be
accepted however that variations do occur and that these may influence the
results.
From the point of view of discards, neither the standard trawl nor the T90
showed any significant level of size related discards over the range of species
sampled.
The relatively low numbers of fish sampled for the catch comparisons make it
difficult to draw conclusions about the relative performance of the T90 and
standard gears. However there are some indications which tend towards
supporting the observations and experiences recorded by Icelandic fishermen
in that the T90 configurations appear to catch and retain more of the larger
size grades of fish, particularly haddock. It was evident however that the
larger grades of whiting were escaping from the T90 net. Whiting as large as
50cm were regularly meshed in the T90 codend and large whiting were
observed swimming out of the codend/extension while the net was on the
surface during hauling.
Example of the size of whiting meshed in T90
codend (~50cm)
It is reasonable to assume that if the meshes in the T90 trawl do have a more
consistent mesh opening compared to the standard trawl, then more escapes
could be occurring over larger areas of the main body of the trawl.
From a commercial perspective, the loss of relatively large numbers of
marketable size grades of any commercial species from a T90 net is likely to
be unacceptable if there is not some benefit or advantage to offset this. In the
case of T90, the Icelandic experience has been an improvement in the
numbers of larger fish retained. This observation could not be demonstrated
with any confidence in this trial.
SR595
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© SEAFISH
The summary of catch data in Table (2) does indicate that the combination of
the T90 trawl with a standard codend/extension may be a workable
commercial option where smaller grades of fish are an important component
of the overall catch. Over the four 24 hour sampling periods for which the
data are given for the four net configurations, the catch values indicate that
the T90/standard codend combination came out on top.
No conclusions could be drawn from the observations on catch quality. Here
again there was some expectation based on Icelandic findings that the T90
configuration would show benefits. The T90 codend construction in
combination with the ‘Alaska’ knot for securing the codline was seen by the
crew to have merit from the point of view of allowing more freedom for fish
movement within the codend. This is perceived to be a benefit for fish quality
by reducing the risk of crushing damage associated with large catches.
Research work carried out by SINTEF at the Flume Tank in Hirtshals,
Denmark has demonstrated how improved water flow in and around the
codend can reduce codend oscillation which is thought to help preserve fish
condition within the codend, (6Hansen).
T90 mesh
codend with
‘Alaska’ knot
closure
Images of model codends in the
Flume Tank showing the relative
shapes and potential influence on
catches.
Standard
diamond mesh
codend with
conventional
codend knot
closure
The limited results and observations from these trials can only be considered
as indicative of the relative performance of the two trawl types. Further trials
would be required to confirm the indications observed here.
SR595
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© SEAFISH
6. Further Work
SEAFISH is continuing its programme of work investigating ways of reducing
the environmental impact of fishing gears into 2008/09. This will include
follow up work to this project to examine other gear designs and fisheries that
could benefit from the T90 concept.
In light of the results from this trial it is suggested that the development of this
specific T90 technology would benefit from additional work. It is
recommended that additional fishing trials be conducted with the existing
gear, on a commercial basis, over a longer time frame in a fishery specifically
targeting larger size grades of fish.
There is a requirement to establish more definitive data with respect to the
fuel saving attributes associated with the T90 technology in order to offset
what appear to be the more selective properties of T90 netting.
Further work should include:
•
Further collaboration with Icelandic net makers and introduction of the
T90 concept to UK net makers to aid the development of this
technology
•
Scale modelling and Flume Tank testing of existing and any new
designs developed in order to optimise rigging and performance.
•
Underwater observations of the T90 trawl to better understand how the
technology works and observe fish behaviour /reactions to the T90
gear.
•
Investigation of more specific applications of this technology in directed
squid fisheries.
SR595
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© SEAFISH
7. Acknowledgements
The following people and organisations are gratefully acknowledged for their
participation and contributions to this project.
Hermann Gudmundsson
Jon Einar Marteinsson
Skipper and crew
Michael Humphrey
Mike Montgomerie
LHD Shetland Ltd
Jackson Trawls Ltd
Fjardanet
Fjardanet
MFV Mizpah
SEAFISH
SEAFISH
Lerwick
Peterhead
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© SEAFISH
8. References
(1) Moderhak, W. 2000. Preliminary investigations of the
mechanical properties of meshes turned through 90º.
(2) Moderhak, W. and T. Niemiro 2005. Mechanical properties of
twine and netting used in Baltic fisheries.
(3) Moderhak, W. 2000. Selectivity tests of polyamide and
polyethylene codends made of netting with meshes turned
through 90º.
(4) Moderhak, W. 1993. Some problems of water flow through
trawl codends.
ICES C.M. 1993/B: 11
(5) Hansen, U.J., L.H. Knudsen, P. Nielsen & E.M. Andersen,
1996: Development of fishing gear for Fishing Vessel 2000
Danish Institute for Fisheries Technology and Aquaculture
(confidential)
(6) Hansen, U.J. 2004. Performance of a trawl codend made from
90º turned netting (T90) compared with that of traditional codends.
ICES FTFB WG meeting, Gydynia, April 20 – 23, 2004
SR595
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© SEAFISH
Figure (1): General trawl gear rigging arrangement used during trials
5 fthm
backstraps
30 fthm split
bridles
30 fthm
single
sweeps
5m2 NETS
doors
SR595
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© SEAFISH
Figure (2): Ground Gear specification –‘Rockhopper’ rig
(Shown as half rig)
19’ 10’’x 24mm wire
24 x 18’’ discs
1 x 7’’x 7’’ spacers between discs
6 x 2kg lead washers distributed evenly
across section
Mid-Section
CL
Shoulder Section
Toe Section
19’ 10’’x 24mm wire
12 x 18’’ + 13 x 16’’ discs
1 x 7’’x 7’’ spacers between discs
6 x 2kg lead washers distributed evenly
across section
19’ 10’’x 24mm wire
16 x 16’’ discs
2 x 7’’x 5’’ spacers between discs (except last two)
6 x 2kg lead washers distributed evenly across section
Figure (3): Outline schematic showing relative shape and size of trawls under test
Jackson Trawl
Floatation: 58 x 280mm
Fjardanet T90 Trawl
Fishingline: 41.0m
Floatation: 58 x 280mm
Fishingline: 41.0m
305mm x 5mm
310mm x 4.3mm
305mm x 5mm
310mm x 4.3mm
160mm x 3.1mm
160mm x 4mm
200mm x 4mm
200mm x 3.1mm
160mm x 3.1mm
160mm x 4mm
160mm x 4mm
160mm x 3.1mm
160mm x 3.1mm
160mm x 4mm
120mm x 4mm
120mm x 4mm
130mm x 3.1mm
120mm x 4mm
120mm x 4mm
Note:
Mesh sizes for Jackson Trawl are
shown as inside- mesh
measurement
120mm x 4mm
120mm x 4mm
Note:
Mesh sizes for Fjardanet Trawl
are shown as full- mesh
measurement
130mm x 3.1mm
Figure (4): Codend arrangements for the standard and T90 trawls
60
41
60
40
4mm
Single
120mm
inside
3.1mm
Single
130mm
Full
mesh
100
60
40
60
40
4mm
Single
120mm
inside
3.1mm
Single
130mm
Full
mesh
100
60
60
4mm
Single
120mm
inside
~15
SR595
5mm
Double
120mm
inside
60
100
100
40
40
3.1mm
Single
130mm
Full
50
50
~15
37
5mm
Double/
130mm
Full
40
50
50
© SEAFISH

Reducing Drag in Towed fishing Gears

Transcription

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